Tactical: When damage would be applied to one of this creature's hit dice or armor dice, if the damage does not match the pips on that die, it is ignored. Critical hits ignore the limitations of this ability.
I might consider this as a hi-level PC ability too. In any case, though this monster ability is intended for KIWF, it will work in any system where actual dice are used to track hit points. You could even convert D&D monsters with hit dice listings.